We have demonstrated that: (1) Ovulation can be induced at a predeterminable time by steroid treatment in baboons; (2) by histological examination of the ovaries, it was shown that there was a definite change in the distribution of follicular population in the distribution of follicular population in the treated baboons as hypothesized; and (3) we have measured steroid levels in utero-ovarian vein plasma using a technique developed in our laboratory. The results showed the importance of studying serially in the same animal the ovarian secretion of steroids from both the active and inactive ovary. The proposed work therefore attempts to attack three problems: (1) to obtain a better perspective of the temporal relationship between steroids (estrogen, progesterone, testosterone, and androsteneone) secreted by the nonhuman primate ovary, the peripheral concentrations reached, and gonadotropins (LH, FSH) in peripheral blood; (2) to obtain the details of folliculloesis and its relationship to the hormonal milieu in which this development takes place; and then (3) to modulate this hormonal environment so that desired control of the maturation and ovulation process can be achieved. Modern techniques of RIA already operative in our laboratory, plus the ability to do daily repeated sampling of utero-ovarian blood from the same animal by the use of a laparoscope, now make possible a more sophisticated approach and give a better insight into the dynamics of ovulation. The information obtained will have basic significance; moreover, precise control of ovulation is important for clinical applications, such as improvement of rhythm methods for fertility control, and also in pharmacological studies of pro- or antifertility agents where the time (and therefore the site) of action of new agents must be determined as part of the study of their mechanisms of action.